Process for the conversion of calcium sulfate dihydrate into alpha-hemihydrate

In a process for conversion of calcium sulfate dihydrate, obtained as effluent from flue gas desulfurization units, by wet scrubbing of sulfur dioxide using calcium carbonate as an absorbent, which scrubbing process produces calcium sulfate dihydrate which is separated and a calcium chloride containing scrubbing liquor which is recycled to the scrubbing process wherein said calcium sulfate dihydrate is converted into alpha-hemihydrate by suspending the dihydrate in a salt solution used as a calcination agent, comprising mixing the dihydrate with the salt solution to form a slurry;

• adding seeds;
• executing the conversion into alpha-hemihydrate at elevated temperature and at ambient pressure;
• washing and drying the obtained alpha-hemihydrate;
• and
• the improvement comprising:
• concentrating a portion of the scrubbing liquor to a calcium chloride content of about 20% to 40% by weight as the salt solution;
• adding magnesium chloride and potassium chloride to the salt solution to increase the concentration of the magnesium chloride and potassium chloride in the salt solution to about 2 to 8% by weight magnesium chloride and about 0.5 to 2% by weight potassium chloride;
• mixing said salt solution, said calcium sulfate dihydrate and
• adding about 5% to 30% by weight of seed crystals;
• and
• then heating the resulting mixture to an alpha-hemihydrate conversion temperature of 85.degree. C. to 98.degree. C. and maintaining said temperature for a period of time to form alpha-hemihydrate crystals.